1. Field of the Invention
The invention relates to a method for generating a shell mold for a casting, in which the shell mold is described from a multiplicity of finite volume elements and/or surface elements which are connected to one another via node points and which together form a mesh, the so-called FE mesh, for describing the surface of the shell mold.
2. Discussion of Background
Methods of the generic type form the basis for the industrial manufacture of complicated castings of complex geometry which, since their use often places very considerable loads on the cast material, have to satisfy extremely high demands with regard to the casting quality. Of particular interest in this context is the production of turbine blades which, in the field of gas turbine engineering, are exposed to extremely high material loads at high pressure and temperature conditions. It is unquestionable that the service life of such castings is particularly dependent on the quality of their production, since in casting processes, in addition to the use of precisely formed casting molds, the cooling phase which follows the casting operation also has a considerable influence on the formation of mechanical stresses inside the casting. Both the geometry of the casting and the cooling operation are decisively affected by the design of the shell mold in which the casting itself is formed.
Thus, for the complex production of turbine blades, the production of ceramic shell molds using the so-called investment casting process has become established in the course of the last 25 yeas. This production process, which corresponds to a dipping process, makes it possible to produce shell molds with a variable wall thickness which is a function of the surface curvature of the casting. Starting from a pattern of a turbine blade which is made from a fusible material (e.g. wax), this blade is dipped into a liquid bath comprising refractory material which can solidify, so that a type of coating is deposited on the surface of the turbine blade. After the turbine blade has been removed from the bath, the coating which has been deposited on the surface of the turbine blade is hardened, so that the dipping operation can be repeated sufficiently often to produce a desired shell mold thickness. Finally, the shell mold which has been obtained in the manner described above is heated until the pattern turbine blade material (wax) is liquefied and is able to flow out of the shell mold.
Particular care has to be taken when producing such shell molds, since when the molten material solidifies in the shell mold, the thickness of the shell mold is the decisive factor in determining the heat flux through the turbine blade and therefore the casting quality.
Furthermore, it is possible to generate the shell molds in a theoretical, numerical fashion using finite elements (FE) which are suitable for simulating surface geometries.
To carry out an FE simulation, the casting is made into a mesh on the basis of technical drawings or CAD data. Thus, for the shell mold, detailed geometric data is available only in the main blade area of the turbine blade, and therefore it is possible to generate the shell mold in two separate working steps:
In the first step, a shell mold is generated manually on the main blade area of the blade which has been made into a mesh. There are two possibilities for doing this:
1. Surface elements which correspond to the surface of the shell mold are generated manually. Then, the volume mesh between the surface elements of the pattern casting mold and the manually generated surface mesh of the mold shell is generated automatically.
2. Volume elements are manually applied to the surface elements of the pattern casting mold, which volume elements describe the volume mesh of the shell mold.
These two steps are typically carried out using commercial mesh-generation programs (e.g. IDEAS SUPERTAB, CATIA, . . . ).
Exact compliance with the shell mold thickness in the main blade area requires time-intensive definition of the shell mold surface. In the root and head areas of the turbine blade, by contrast, the production process results in large shell mold thicknesses which are determined by the complex surface in this area. The large shell mold thickness in the root and head areas of the blade often results in surfaces of complex structure. These numerous complex surfaces cannot be manually machined from the surface geometry of the pattern. Therefore, complex geometries are simplified, to the detriment of the accuracy of the geometric description of the shell mold. In this case, commercial software is used to estimate the position and shape of the surface of the shell mold, which are correspondingly inaccurately described. Using the commercial software, firstly a surface element mesh of the shell mold is generated from the estimated, and therefore inaccurate, surface description of the shell mold. Then, a volume element mesh is automatically generated from the surface element mesh of the pattern and the shell mold.
By using automatic shell mold generators, on the basis of methods for producing three-dimensional objects using finite elements, as is disclosed, for example, from American patent U.S. Pat. No. 5,581,489, it is possible to considerably reduce the mesh-production outlay involved in the generation of shell molds for producing a gas turbine blade. It is thus possible, when producing the mesh for a gas turbine blade, to save roughly up to two man weeks compared to the traditional method of producing a mesh. At the same time, the accuracy of the details of the geometry description of the shell mold, and consequently the calculation accuracy of the finite element simulation, increase.
Due to the considerable potential for saving time when producing the mesh for shell molds, a number of attempts have already been made to develop automatic shell mold generators using finite element or control volume meshes. However, the known shell generators have the drawback that the shell generators do not take into account all aspects of the production process, in particular the dependency of the shell mold thickness on the curvature. Consequently, however, exact simulation of the precision casting process is not possible, since commercially available mesh generators do not take into account either the curvature or the orientation of the volume elements of the shell mold. Consequently, the accuracy of the temperature, stress and deformation simulation, and of the flow simulation of the casting and shell mold geometry, decreases significantly.
A key factor for the further development of gas turbine blades lies in the castability of the blade itself, a problem which, owing to the ever more complicated shapes of blades, is increasingly becoming the focus of gas turbine development. Conventional casting technology nowadays no longer allow relatively large blades to be cast without flaws.
The casting operation can only be evaluated and optimized by using numerical simulation of the casting process. However, optimization of a precision casting process requires from two to twenty variations or optimization steps in order to obtain a suitable geometry for the cast component and thus the ideal shell mold mesh. Therefore, rapid and accurate production of a mesh for the shell mold typically contributes 4-10 man weeks to reducing the development time for the casting process.
Accordingly, one object of the invention is to develop a method for generating a shell mold for a casting, in which the shell mold is described from a multiplicity of finite volume elements and/or surface elements which are connected to one another via node points and which together form a mesh, the so-called FE mesh, for describing the surface of the shell mold, in such a manner that the curvature characteristics of the casting are also taken into account when determining the shell mold, in that the thickness of the shell mold is to be designed as a function of the curvature characteristics of the casting and therefore of the shell mold.
In order to be able to individually modify the level of accuracy with which the shell mold is matched to the surface of the casting, the shell mold mesh on which the method is based is to be made coarser or finer. Furthermore, there should be possibilities for correction during the generation of the shell mold mesh, so that holes or recesses on the surface of the casting can be better taken into account, for example by closing holes, and it is also intended for it to be possible to recognize and eliminate impermissible volume elements which, for example, overlap one another. Finally, it is to be possible for the shell mold generator also to be used for other methods which require curvature-dependent layers.
The object on which the invention is based is achieved in accordance with claim 1. Features which refine the inventive concept form the subject matter of the subclaims.
According to the invention, a method in accordance with the preamble of claim 1 is refined by the combination of the following method steps:
Firstly, the real surface geometry of the casting is recorded and generated in the form of a coherent base mesh which is composed of surface elements which are defined and connected by node points. Auxiliary programs which are known per se and allow the FE geometry of the casting which is to be cast to be read in are suitable for this purpose. These data can also be imported from CAD systems, provided that the casting was designed on the basis of a CAD system.
Then, an approximated simulation of the real surface geometry of the casting geometry is produced by means of finite volume elements and/or surface elements, which form the FE mesh. The base node points which are contained in the base mesh and are assigned to one or more surface elements together, linked with the associated surface elements, define the surface of the casting mold.
Furthermore, for all base node points, so-called shell vectors are generated, which are oriented perpendicular to each of the surface elements which bear against the base node point and the length of which is in each case generated as a function of the curvature characteristics at the base node points and corresponds to the distance to a new surface element which is positioned above each base node point and in each case defines one volume element and/or new surface which, together with all the other newly formed volume elements and/or surface elements, leads to a first layer of the shell mold.
The end point of the shell vector imposed on the base node point defines the new shell node point.
By means of iterative repetition of the generation of shell vectors on base node points, taking into account the topology of the base node points, the newly generated shell node points from the previous generation step again defining a base mesh, the associated volume elements and/or surface elements of which define a further layer of the shell mold, the complete shell mold is gradually produced in the form of an onion-skin structure, which shell mold reaches a desired, predetermined final thickness as a function of its local curvature.
To form in each case one layer of the shell mold on the basis of the shell node points and the shell vectors which are to be formed as a function of the curvature characteristics, two differently operating modules are used:
While in the Mapped Mesher new volume elements are generated directly in order to form a shell mold layer, in the Free Surface Mesher only the new surface elements are generated. However, the generation of new elements in the Mapped Mesher can only be achieved if the surface is convexly curved (curved outward). A slightly concave curvature is only permissible if the radius of curvature is greater than the local shell mold thickness. If deformed elements are formed during the element generation, the shell mold generation is terminated in the Mapped Mesher.
In the case of the Free Surface Mesher module, the volume between the casting surface and the calculated shell mold surface is made into a mesh using tetrahedrons by means of commercially available software. This step is not necessary for the Mapped Mesher method, since new volume elements are generated directly at each extrusion step, i.e. at each formation of a new layer.
The following text describes, by way of example, the method sequence for the shell mold generation using the Mapped Mesher method.
The FE model which has been obtained using CAD data or has been synthesized by means of suitable auxiliary programs forms an approximated representation of the real geometry of the casting, for example the geometry of a turbine blade. Thus the casting volume is to be made discrete as accurately as possible by combining many individual volume elements, for example in the form of hexahedrons, pentahedrons and/or tetrahedrons. The corners of these elements, i.e. the so-called node points, determine the position of the individual elements and the three-dimensional links between them. Together, these elements and the associated node points form the so-called FE mesh (finite element mesh).
Since a shell mold around a body is determined not by its volume but by its surface, its surface in the FE model is represented by means of surface elements.
The smallest unit of the FE mesh is the node point. For each base node point of the base mesh, which corresponds to the casting surface or a shell mold surface which has already been generated in a previous step (iterative generation of the mold shell), the method according to the invention, with the aid of the directly adjacent node points, determines a shell node point. The shell node point defines a surface element of the shell mold with respect to each element of the base mesh. Moreover, each element of the base mesh, together with the associated surface element, defines a volume element of the shell mold. Together, these determined volume elements form the primary volume mesh or the first layer of the shell mold.
The primary volume mesh on a concave surface may therefore contain unusable elements. These are volume elements which intersect one another. With this in mind, the method is able to recognize invalid elements and intersections between allowed elements and to eliminate the invalid ones. The corrected shell is also to maintain a predetermined span of curves.